ACAPULCO, Mexico — Mexican schools appear increasingly vulnerable to the country's drug violence, with five human heads dumped outside one school and threats of a grenade attack on another in the past week alone.

From northern border areas to Acapulco, on the Pacific coast, to the port of Veracruz, on the Gulf of Mexico, the trend has seen parents keep their children at home as both students and teachers see themselves as targets.

The five decomposing heads were found near an elementary school Tuesday in the port city of Acapulco, where teachers have been on strike for nearly a month to seek better security and protest extortion attempts on their salaries.

In Veracruz -- where 49 bodies were dumped on roads within three days this month -- parents are increasingly hesitant to send youngsters to class, fearful of armed clashes nearby between the Zetas drug gang and a mysterious new group called the "Zeta Killers."

Hundreds of parents rushed to take their children out of schools in Tierra Blana, in Veracruz state, after warnings of heavy police deployments spread on social networks this week.

Beyond threats linked to drug gangs, violence threatening children and teachers has also occurred in recent weeks inside schools, including in northeastern Sinaloa and northern Nuevo Leon states.

In Culiacan, capital of Sinaloa, teachers protested Thursday in front of the state Congress to call attention to growing theft and attacks in classrooms.

"The community has organized itself and decided not to send children to school until we receive promises from the authorities," said Lourdes Sarabia, director of the National Union of Education Workers of Culiacan.

The mayor of the town of Santiago, in the northern border state of Nuevo Leon, called for calm Friday after messages threatening grenade attacks on schools spread fear through the population.

"It's logical that people panicked. Fear spreads after the appearance of four banners announcing grenades would be thrown at schools," Vladimiro Montalvo told AFP.

"We've asked for help from the army and the police, who are patrolling in the area."

Around a dozen men were detained in relation to the banners on Friday.

The fears appear excessive but are "part of the deterioration of daily life in some communities, as violence affects civilians in public places," according to Javier Oliva, an expert in security issues at the National Autonomous University of Mexico (UNAM).

At the end of August, armed men even shot on parents waiting for their children in front of a school in the notoriously violent norther border city of Ciudad Juarez, killing a teenager and injuring four women.

The challenge in schools was shown months earlier at the end of May, when a teacher in the northern city of Monterrey received an award for distracting a class of five and six-year-old pupils by making them sing as a shootout occurred nearby.

Perhaps the biggest drama though has played out in violence-plagued Acapulco, where thousands of teachers have demonstrated and almost 200 schools in the area have been paralyzed by a month of strike action to persuade authorities to improve security amid extortion threats.

Teachers and local government officials this week came to a deal to return to classrooms on Monday, but it was unclear if the unions would give up the negotiations.

Two weeks ago the government said classes would resume, after they promised to install panic buttons in schools and police patrols nearby, but the protest continued.

Acapulco street seller Elizabeth Garcia, a 26-year-old mother of two, said she felt calmer keeping her kids at home.

"I don't know if it's better that they don't go to school, but at least I know where they are," Garcia said.